The present invention relates to an integrated structure microcomputer provided with a non-volatile RAM memory.
Integrated microcomputers usually comprise a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM) and several other more or less optional functions, such as input/output etc.
For some applications in which a microcomputer is used as a processing and/or control unit, it is necessary to retain some stored information when, for different reasons or necessities, the power supply is cut off.
For this purpose, there have recently been inserted inside microcomputers some non-volatile memory elements, so called EEPROM (electrically erasable programmable read-only memory), which are dedicated to the storage of the information which must be kept.
However, the use of non-volatile memories of the EEPROM type imposes some important limitations.
(a) Every time one wants to store new data, it is necessary that the microcomputer executes a particular sequence of operations to cause the storage. Such operations require a certain execution time (typically, about 10 ms), which limits the microcomputer's speed. Therefore, the microcomputer is obliged to slow the execution of its instruction program, and it is possible to store in a given time only a limited quantity of information. PA1 (b) As non-volatile memory cells can support only a limited number of modifying cycles of their electric state (for example 10,000 cycles), the storage of data in such memory cells, if effected during the normal execution of the program (that is, before the necessity for non-volatile storage due to power cut-off), causes useless ageing of the memory. PA1 (c) On the other hand, if the information processed by the microcomputer is stored in the non-volatile memory only at the moment of power turn-off so as to avoid any speed reduction of the microcomputer during the normal operation and to limit the ageing of the memory cells by modifying their electric state only when it is necessary to save the information, then it is necessary to detect the power drop and to support the voltage level, by means of capacitive elements for the time necessary for storage, that is, for a time which is longer as the quantity of information to be stored is greater and for which greater cost and complexity of the feed circuitry is required. Furthermore, it is also always necessary in such a case to include in the program a special operations sequence.